This invention relates to a process for the preparation and application in situ of a ready to use blend of structural material, the constituents of which blend comprise binder, filler additive and water.
Structural elements according to the invention are mortar or plaster for joining bricks, coating masses for walls, floors and ceilings, joining masses to fill out gaps and adhesives for tiles and similar structural elements. Powdered or granulated dry mixtures from which ready to use blends of structural material can be made by simply mixing them with water are well known and commercially available. Such dry mixtures basically consist of three main components:
binders, i.e. dry substances which, by chemical and/or physical reaction with water turn into a coherent dry mass; PA1 fillers which by themselves do not react with water but are embedded into the binder mass, thus giving additional volume, and which may improve the physical strength and, if desired, thermal or sonic insulation properties of the mass; PA1 Additives, principally meant to improve the physical properties of the wet blend, such as its fluidity, water retention power and setting speed, but in some cases also to influence properties of the hardened material, such as water repulsion or pore volume.
Well-known binders for structural materials are cement, gypsum, quick lime, magnesite or mixtures of them. Commonly used fillers are sand, powdered rock, crushed marble or dolomite, ground slag, fly ash, clay and china clay. Furthermore porous materials like expanded baked clay or foamed polystyrene may be used to improve thermal or sonic insulating power. Additives, usually representing at most 3% by weight, preferably less than 1% by weight of the structural blend as applied, are mainly natural or synthetic polymers such as animal or vegetable glue, alginates, soluble starch ethers, cellulose derivatives such as carboxymethyl cellulose, polyvinyl alcohol, derivatives of polyacrylic acid, e.g. polyacryl amide. Another class of additives are surfactants such as fatty acid esters and sulfonates, alkyl aryl sulfonates, lignin sulfonates and soaps. Still another class, mainly used as setting retardants, are certain acids such as tartaric acid, citric acid or maleic acid. Silicones, which are soluble in organic solvents or mixtures of water and organic solvents can be used to improve the water repellency of the structural surfaces.
Part of the additives referred to above are hygroscopic. This means that mixtures containing them are difficult to keep dry and therefore have to be protected by expensive packaging. Broken packages, especially on building sites may rise extra problems. Structural blends containing as little as 1% by weight of water will already harden in a short time and thus become useless.
Another kind of problem originates in the use of a certain type of machines frequently used in the building industry. Such machines as the well known spray rendering or spray plaster machine serve to handle large quantities of structural blends by mixing and immediately applying them on the building site. These machines usually comprise a mixing chamber with a filling hopper for the dry mixture and a device for water dosage into the mixing chamber. The mixing chamber is connected to a conveyor pump which transports the wet blend via a hose to a nozzle for the application in situ. These machines may work batchwise or preferably in a continuous mode. In both cases the mixing has to be very rapid in order to avoid any interruption in the feeding of the applicator nozzle. Normally only 6 to 20 seconds are available for the complete process of mixing the dry blend with water.
One drawback of the prior art is the high standard of effectiveness for the mixing device: On the one hand the initial dryness of the material requires a very high energy input, resulting also in high wear on the machine parts. On the other hand, on account of the small concentration of additives, an exceptionally high standard of homogeneity of the mixture is required. Additional problems arise by the low rate of dissolution of high molecular additives, such as carboxmethyl cellulose or polyacryl amide, especially if they are present in granulated form. In many cases the time available for mixing does not suffice for complete dissolution of the additives. To overcome this problem the dissolution rate has to be increased by fine grinding or by special treatment of the additive, or even compensated by overdosage, which, of course, is not economical.
A method of sealing and strengthening of a cracking and deteriorating wall surface, by mixing dry plaster of Paris with the necessary amount of water in a nozzle through which the mixture is sprayed onto the surface has been described in German Offenlegungsschrift No. 2,733,540 by Walter D. Kobeski (U.S. patent application Ser. No. 709,699 filed July 29, 1976)
In the Swiss Pat. No. 595 065 a technique is described, by which the problems associated with slow dissolution of high molecular additives can be overcome. This is achieved by adding the additives in the form of an aqueous dispersion, in which they are present in the form of very fine solid particles. The aqueous phase of the dispersion contains dissolved salts or organic solvents in order to prevent the dispersed phase from being dissolved. Such dispersions exhibit the advantage of low viscosity compared with aqueous solutions of corresponding concentration. However, as the salts and organic solvents cannot be removed during the mixing process and thus remain in the finished mixture, they still impair the complete dissolution of the additives. The problem of poor utilisation of the additives therefore still remains unsolved.